I think I agree with Thekingisdead, as in I think he's talking about the route I do up there. It's featured in a few mags, including MBR from a few years back (killer loop).
Nant yr Arian is my favourite trail centre for the reasons already mentioned and that trail has all the best examples. The Syfydrin route is good but you're right, there is a lot of fire road and tarmac involved – the other route follows the best bits of that, then peels off into the forest rather than dropping down the valley on the road only to climb back up the other side.
Definitely best done on a nice day though – better views and it's pretty exposed in places.
If you fancy it, you'll need a map/GPS but drop me an email and I'll send you the details.